Steam reforming

ABSTRACT

The endothermic reaction of a hydrocarbon feedstock with steam and/or carbon dioxide is carried out using a double-tube reactor. In one preferred form of the reactor the catalyst is present as a coating on the outside of the inner tube. In another form inner tubes are mounted in one tube-plate and the other tubes in a second tube plate and the tube plates are disposed across a cylindrical shell so as to define a heat exchange zone (provided with a heat source) a reactants feed zone and a products offtake zone. The heat source is preferably a burner, to be fed with the product of the endothermic reaction, followed by a secondary reforming catalyst. The apparatus makes possible processes for making raw hydrogen-containing gases with advantageous internal heat recovery.

This is a division of application Ser. No. 591,328, filed Mar. 19, 1984,which was abandoned upon the filing hereof.

This invention relates to reforming of hydrocarbons with steam and/orcarbon dioxide and in particular to an apparatus and process forcarrying it out.

In our published European application No. 21736 we describe a processfor producing a gas containing hydrogen which comprises reacting in thegaseous phase a hydrocarbon, hydrocarbon derivative or carbon monoxidewith steam and/or, where appropriate, carbon dioxide, in the presence ofa catalyst supported on a metal or alloy resistant to the conditions inwhich the reaction is carried out; and more particularly a process forproducing a gas containing hydrogen which comprises reacting in thegaseous phase a hydrocarbon or hydrocarbon derivative with steam and/orcarbon dioxide, in the presence of a catalyst with a catalyst outlettemperature such that the product gas contains at least 30% v/v ofhydrogen on a dry basis, in which the catalyst units comprise a supportand at least one active metal selected from the group consisting ofnickel, cobalt and the platinum group metals, characterised in that thecatalyst support comprises a primary support made of a metal or alloyresistant to the conditions in which the reaction is carried out and asecondary support which is a layer of a refractory oxidic materialadhering to the surface of the metal or alloy.

We have now realised that the use of the catalyst supported on the metalor alloy makes possible a new design of reforming apparatus and a newprocess sequence, which apparatus and sequence conveniently but notnecessarily include the said catalyst.

According to the first aspect of the invention an apparatus for theendothermic reaction of a hydrocarbon or hydrocarbon derivative withsteam and/or carbon dioxide comprises at least one outer tube open atone end and closed at the other, means to heat the outer tubeexternally, at least one inner tube within the outer tube and having anopen end near the closed end of the outer tube, means to feed reactantsto the space between the outer and inner tubes and to withdraw productsfrom the inner tube, and a steam reforming catalyst in the space betweenthe inner and outer tubes: characterised in that steam reformingcatalyst is present as a layer adhering to the outer surface of theinner tube.

To afford adequate geometric surface for the steam reforming catalystthe outer tube can be of smaller diameter than has been common in steamreforming practice: up to 75 mm I.D., with an inner tube correspondinglysmall, leaving an annular space 5-20 mm wide, would be suitable.Possibly a larger outer tube, up to for example 150 mm I.D. can be usedwith a plurality of inner tubes. Preferably the inner tube has anextended surface, provided for example by fins or attached spikes orwinding wires. The surface area is typically 3 to 10 times that of thesmooth tube. A very suitable extended surface is provided by a multi(e.g. 6-10) start helix with in all 80 to 400 turns per m. The fins orhelices can be continuous or interrupted.

The inner tube can contain a turbulator, to improve heat exchangebetween product gas and the reactants in the annular space. Neither theinner surface of the inner tube nor the turbulator should carrycatalyst, of course.

The catalyst layer can itself be a catalyst by virtue of the choice ofmaterial of construction or of chemical or physical modification of itssurface for example by cold-rolling of a nickel-containing unit. Morecommonly it is a support for active material, for example one or moremetals from Group VIII of the Periodic Table. Especially when thesupport has a very low adsorptive surface, for example when it is ametal or alloy, it (as "primary support") carries a coating ofadsorptive material ("secondary support") and the active material isassociated with that coating. Preferably the Group VIII metal content,if non-noble, is in the range 30-60% ^(w) /w calculated as equivalentNiO on the total coating. Such adsorptive material typically has a porevolume over 0.2 cm³ g⁻¹ and a surface area of at least 1.0, preferablyover 15, especially in the range 50-200 m² g⁻¹. The secondary supportpreferably has a thickness in the range 0.01 to 0.3, especially 0.02 to0.1 mm.

The secondary support typically comprises alumina, especially gamma oreta alumina. Other sesquioxides, for example, chromia and rare earthoxides may take up at least part of the secondary support. Other usefulsecondary support oxides are titania, zirconia, hafnia, thoria, vanadia,urania, oxides of manganese, molybdenum and tungsten and combinedoxides.

Preferably the secondary support includes a grain growth inhibitor, forexample at least 0.05, especially 0.1 to 5.0% by weight of yttrium or ofone or more rare earth oxides, especially of cerium, or praseodymium.

When nickel and/or cobalt are present in the catalyst, it is expectedthat the secondary support, if it contains a sesquioxide, will include,at least after a period of process operation, some nickel and/or cobaltspinel. It is within the invention to have the secondary supportmaterial at least partly in the form of spinel, whether of nickel and/orcobalt or of a divalent metal having a difficultly reducible oxide,especially magnesium or manganese or, less preferably, zinc. Sincenickel and/or cobalt present as spinel is in a reduction-resistantoxidic form, it does not contribute significantly to the activity of thecatalyst: active nickel and/or cobalt are additional thereto.

In a catalyst comprising nickel and/or cobalt there may also be presentone or more platinum group metals, which are capable of increasing theactivity of the nickel and/or cobalt and of decreasing the tendency ofcarbon lay-down when reacting hydrocarbons higher than methane. Theconcentration of such platinum group metal is typically in the range0.005 to 1% as metal, calculated on the coating. Further, the catalyst,especially in preferred forms, can contain a platinum group metal but nonon-noble catalyst component. Such a catalyst is more suitable than oneon a conventional support because a greater fraction of the active metalis accessible to the reacting gas. A typical content of platinum groupmetal when used alone is in the range 0.005 to 5% ^(w) /w as metal,calculated on the coating.

The specific surface of the catalytic metal is suitably in the range 1to 500 m² /g of coating. Within these ranges the larger areas arepreferred for reactions under 600° C.

When both non-noble and noble metals are present a useful level ofcatalytic activity can be obtained using a notably small concentrationof such metals, namely under 2%, especially 0.01 to 0.5, % ^(w) /w inall, calculated on the total of secondary support and such metals. Thepreferred noble metal is rhodium.

The catalyst can be made by applying a compound of the active metal andthe secondary support together to the primary support. In a preferredmethod the secondary support is applied to the primary support, thecombination is preferably calcined, and then a solution containing athermally decomposable compound of active metal is applied. When it isdesired to regenerate the catalyst, the inner tubes can be withdrawn andre-treated with a compound of the active metal.

As an alternative to the apparatus just described the invention providesa catalyst unit removable from the space between the outer and innertubes.

According to the second aspect of the invention a contact material is inthe form of tubular units having perforated walls and means to distancethose walls from the walls of a surrounding container in which they areto be stacked in axial relationship with one another and with thecontainer.

The invention provides also such a container charged with such units,which are so distanced from the container walls as to afford a space forfluid flow in contact with the container walls.

The means to distance the container walls from the unit walls can beintegral with the unit walls (in which event the unit is a new article)or can be provided by spacers around or stacked between units that neednot themselves have integral distancing means.

The purpose of the perforations is to permit fluid flow in at least onedirection transverse to the general direction of flow through thecontainer. This is especially valuable when contact with the unitsinvolves absorption or evolution of heat, and thus the inventionincludes a heat exchange apparatus including such a charged container,especially a tube and means for heat exchange through the containerwalls. Examples of such heat exchange apparatus are a steam reformingfurnace, an ammonia cracker and a heavy hydrocarbon cracker (allendothermic processes) and a methanator, ammonia synthesis reactor,methanol synthesis reactor, aromatisation reactor, ammonia/waterabsorber or ammonia/acid reactor (all exothermic processes).

The perforations in the unit walls typically amount to between 20 and60% of the unit wall area. For many purposes, but especially ahydrocarbon steam reforming process at for example 10-60 bar abspressure, the smallest dimension of the perforations is at least 1 mm,and preferably is in the range 2-5 mm.

Especially when, as is preferred, the units are made of metal or alloy,the perforations are the result of punching metal foil or sheet, andpreferably each such punching leaves at least one internal or externalvane attached to the unit wall; such vanes increase the geometricsurface of the unit and can, if suitably pitched, ensure better contactwith the container walls and thus improved heat transfer.

The distancing means typically define, in cross-section, a circumscribedcircle having a diameter between 1 and 30%, especially between 5 and20%, greater than that of the unit. If such means is integral with theunit, there are at least 3 projections to ensure equidistance fromcontainer walls, and such projections may occur at least at or near bothvertical extremities of the unit, to ensure co-axial stacking;alternatively the units may interlock, so that one extremity of each islocated by the neighbouring unit; or only alternating units in a stackneed be externally projectioned. In an extreme case the outer wall ofeach unit can be corrugated, that is, be formed with a close successionof projections.

If spacers are used, these can be for example other tubular units ofgreater diameter having internal projections on which the distancedunits rest. Such other units may fit snugly or loosely in the container.Alternatively the spacers can be shorter tubular units or evenessentially planar, for example, toothed rings or non-slip washers.

The above-mentioned internal vanes, or possibly other internalprojections, can also serve to distance the units from an internal heatexchange surface or from other units stacked inside them.

The units can, if desired, be linked together axially, for example instacks of 2 to 100, to facilitate charging to a tubular container.

The invention in its second aspect provides further a catalyst supportin which each unit is made of highly calcined ceramic or of metal oralloy and carries a layer of catalyst support material as alreadydescribed, a catalyst precursor in which such support carries a compoundconvertible to catalyst by reduction and/or sulphidation, and theso-formed catalyst. Such a compound is typically of one or more metalsfrom Groups Ib, V, VII or VIII of the Periodic Table especially asdescribed above. If desired, any spacers used can also be coated withsuch catalyst support and/or catalyst precursor or active material.

The invention provides also chemical processes carried out in suchapparatus or over such catalysts. In particular the reaction of ahydrocarbon with steam and/or carbon dioxide to produce a gas containingat least 30% ^(v) /v of hydrogen on a dry basis is operated at 550-1000°C. and at 1-60 bar abs pressure. In a typical process of this kind thegeometric surface of the catalyst is well below the level of about 300m⁻¹ common when using conventional ceramic ring catalyst, and is, forexample in the range 40-200 m⁻¹. As a result the pressure drop can beunder 10%, for example 0.1 to 2.0%, that of such conventional rings.Catalyst units to be used are for example 40-160 mm in diameterexcluding external projections or spacers.

For such processes the hydrocarbon feedstock preferably has a boilingpoint not over 220° C. and is most conveniently normally gaseous, andespecially has a hydrogen to carbon atomic ratio of at least 3.5. If ahydrocarbon derivative is used it is most conveniently methanol orethanol.

According to a third aspect of the invention an apparatus for the saidendothermic reaction comprises

an outer shell;

mutually parallel first and second tube plates disposed across the shelland dividing it into three successive zones, namely a heat exchangezone, a reactants feed zone and a products off-take zone;

at least one relatively wide tube extending into the heat exchange zonefrom the first tube plate bounding that zone and closed at its extremitywithin that zone;

at least one relatively narrow tube extending from the second tube plateinto the extremity of each relatively wide tube; and

a steam reforming catalyst in the annular space between the tubes.

The outer surface of the relatively narrow tube preferably carries anadherent layer of the catalyst, but it can carry catalyst unitsmechanically linked to it, or there can be structured, especiallytubular units according to the second aspect of the invention, or loose,catalyst in the annular space.

Especially if the adherent catalyst is used, each relatively wide tubepreferably has its closed extremity upwards, for these reasons:

(a) piping connections are facilitated;

(b) removal of the inner tube to permit catalyst replacement isfacilitated;

(c) the space between the outer tubes can be packed, thus improving heattransfer;

(d) use of secondary reformer gas as heat source can be facilitated, aswill be described below.

The outer shell preferably withstands superatmospheric pressure,especially in the range 5-120, for example 25-80, bar abs.

In the apparatus according to the first or third aspect of the inventionthere is provision to heat the outer (wide) tubes externally. This canbe for example one or more fuel burners within the external shell: inthis event, to avoid excessive flame temperatures and production ofoxides of nitrogen, two such burner systems can be used in series, thefirst fed with a substantial excess of combustion air and the second fedwith the oxygen-containing effluent of the first. Either system, butmore especially the second, can include a combustion catalyst. When twosuch burner systems are used in series, the reaction tubes arepreferably operated in parallel, especially when the product is to bemethanol synthesis gas or hydrogen.

In another example the source of heat can be the effluent from a hightemperature chemical reaction step, especially from a catalytic"secondary reformer" in which the product gas from the endothermicreaction is reacted with oxygen to decrease its methane contact. Such asecondary reforming step can be carried out in a separate vessel; butvery suitably the heat exchange zone contains, upstream of the tube, abed for a secondary reforming catalyst and, upstream of that bed andspaced from it so as to avoid damage to the catalyst a secondaryreformer burner, in which primary reformer gas and oxygen, possible asair or enriched air, are brought together and reacted in a flame.

In a further example the source of heat is a gas heated in a nuclearreactor.

When the source of heat is combustion of fuel, this is carried outpreferably at superatmospheric pressure and the combustion gas isexpanded in an engine to provide useful power. The combustion pressurepreferably differs from that of the reactants inside the tubes by notmore than 20 bar

The invention provides also a set of processes for producing rawhydrogen streams.

In particular the invention provides a process for producing raw ammoniasynthesis gas by the steps of

(a) primary reforming a hydrocarbon feedstock with steam to give a gascontaining carbon oxides, hydrogen, methane and unreacted steam;

(b) reacting the product of step (a) catalytically with anoxygen/nitrogen mixture to decrease its content of methane and introducenitrogen; and

(c) obtaining part of the endothermic heat required

for step (a) from the hot effluent of step (b); characterised bycarrying out step (a) in 2 stages, the first heated by the hot effluentof step (b), the second heated from an external heat source.

Such a process differs from previous proposals, in which the two stagesof step (a) are to be carried out in the opposite order. The reason forthis was that previously available catalysts had little activity atbelow about 650° C. and therefor required the very intense heating fromthe external heat source at the start of reaction, at which thereactants partial pressure is high and the endothermic heat demandcorrespondingly high. In such processes recovery of heat from theeffluent of step (b) was by heat exchange with gas that had alreadyreacted to a substantial extent in step (a); therefore the temperaturedifference was small and much heat had to be recovered by other means.By using the process of the invention the effluent of step (b) can becooled to 450° to 600° C., resulting in substantially greater heatrecovery within the reforming steps than was previously convenient. Lessheat need be recovered as high pressure steam, the demand for which isin any event less in recently developed ammonia production processesincluding steam reforming at relatively low steam ratios and limitedsynthesis gas compression.

The second part of step (a) can be carried out in a pressure furnace oran ordinary steam reforming furnace fired at atmospheric pressure.

In the process the oxygen/nitrogen mixture can be air and can be fed ata rate producing after shift and CO₂ -removal, a synthesis gas having anH₂ :N₂ ratio approximately stoichiometric (usually 2.5-3.0). If desired,the ratio can be lower, as for example with nitrogen removal fromreacted synthesis gas in the process of our European patent No. 993, orwith nitrogen removal before the gas enters ammonia synthesis. If carbonoxides are to be removed by methanol synthesis, the ratio is chosen tosuit the relative outputs of methanol and ammonia.

Alternatively raw hydrogen, or a raw synthesis gas for conversion toorganic compounds, is produced by the process modified by feeding anoxygen-rich gas (over 80, especially over 95% ^(v) v) at step (b).

The invention also provides processes in which the sources of heat areentirely external and heat recovery from hot gases is conventional, forexample by steam superheating and steam raising and preheating of boilerfeed water.

Corresponding to the third aspect of the invention the inventionprovides also a process for producing a raw hydrogen stream convertibleto ammonia synthesis gas or hydrogen-rich gas or organic compoundssynthesis gas, by the steps of

(a) feeding a mixture of a hydrocarbon feedstock with steam into aplenum zone and therein heat exchanging it with a hot stream to bedescribed in tubes to be described surrounded by that zone;

(b) feeding the resulting heated mixture into a plurality of annularblind heated catalyst zones each surrounding one of the said tubes andeach heated externally in a heat exchange zone by a medium to bedescribed, and reacting the mixture therein to produce a gas containingcarbon oxides, hydrogen, methane and steam;

(c) feeding the resulting reacted mixture back through the said tubes inheat exchange with the reacting mixture and then as the said hot streamwith the mixture in the plenum zone, whereby partly to cool the reactedmixture;

(d) collecting the partly cooled gas in a products offtake zone, feedingit to a burner, partially combusting it with oxygen as such or as air oroxygen-enriched air (depending on the intended nitrogen content of theraw hydrogen stream to be produced) and passing the combustion productover a catalyst whereby to decrease the methane content thereof, thesaid burner and catalyst being disposed in the heat exchange zonespecified in step (b);

(e) passing the catalyst effluent as the said medium in heat exchangewith the exteriors of the said annular catalyst zones.

In any such processes there follow steps of cooling and removal ofexcess steam as liquid water. When the product is to be ammoniasynthesis gas or hydrogen-rich gas, the raw gas is subjected also toshift, CO₂ -removal and fine purification from traces of carbon oxides.

The invention is illustrated by the accompanying drawings, in which

FIG. 1 shows in diagrammatic vertical section two preferred forms A andB of apparatus according to the first and third aspects of the inventionand a preferred combination thereof;

FIG. 2 represents a unit according to the second aspect of the inventionin position in a container which is a heat exchange tube;

FIG. 3a is a section on line 3a--3a in FIG. 2;

FIG. 3b is a section on line 3b--3b in FIG. 2;

FIG. 4a is a side view of an alternative unit according to the secondaspect of the invention; and

FIG. 4b is an axial view of the unit shown in FIG. 4a.

In FIG. 1 the full line paths labelled P refer to the combination anddotted paths labelled Q and R to independent operation of A or B. Forconvenience, integers of both apparatuses are indicated by correspondingnumbers, those in B exceeding those in A by 100. The points ofdivergence of the paths P, Q and R are indicated by numerals 201, 202and 203.

The outer shell consists of cylindrical centre portion 10, 110, upperportion 12, 112 and lower portion 14, 114. Upper portion 12, 112 isformed with a mixing and burner zone 16, 116, to which is connected afuel inlet 18 or gas inlet 118 and an air inlet 20, 120, and is joinedby flange 22, 122 bolted to cylindrical centre portion 10, 110. Theupper portion 112 of apparatus B also contains secondary reformingcatalyst bed 113.

Centre portion 10, 110 is divided horizontally by tube plate 26, 126 thespace above which constitutes the heat exchange zone 24, 124 in virtueof the "outer" heat exchange tubes 28, 128 extending upwards from thetube plate with closed upper ends. Zone 24, 124 includes a packed region30, 130 to improve heat exchange between hot gases outside and reactantsinside tubes 28, 128, and an outlet which (32) in A is for flue gas and(132) in B is for cooled secondary reformer outlet gas.

Beneath tube plate 26, 126 is reactants feed zone 34, 134 bounded bytube plate 36, 136, which extends outwards to form a flange bolted toflange 38. Tubes 40, 140 each having an extended, catalyst-coatedsurface 42, 142, extended upwards from tube plate 36, 136 almost to theclosed end of tubes 28, 128. The inner surface of outside tubes 28, 128may or may not have a catalyst coating.

Beneath tube plate 36, 136 is product offtake zone 48, 148 from whichoutlet 50 in A leads to gas inlet 118 in B, and outlet 150 in B leads togas inlet 46 in, A.

Outlet 32 in A leads to gas turbine 52 which exhausts at 54 to low gradeheat recoveries (not shown) and which drives combustion air compressor,56 feeding air inlet 20 and alternator 58.

The process for producing raw ammonia synthesis gas by the first processaspect of the invention can follow path P in the apparatus combination.A mixture of desulphurised natural gas and steam at for example200°-450° C. is fed at 146 of B to reactants feed zone in which it isheated by heat exchange with the gas in tube 140. It passes up theannular space in contact with catalyst 142 receiving heat both frominner tube 140 and from heat exchange zone 124, until at the top itstemperature is for example 650°-750° C. and its methane content is forexample 20-'% ^(v) /v on a dry basis. It then passes down inner tube 140as a source of heat for incoming gas and leaves the bottom portion 114of the shell by way of products offtake zone 148 and outlet 150 en routefor reactants inlet 46 of A. The source of heat in zone 124 of B will bedescribed below.

In A the path of the gas is as in B but when in contact with thecatalyst, the reactants are heated more strongly, to a final temperatureof for example 800°-850° C., by combustion of fuel fed at 18 with hotair fed at 20, part of the heat exchange being radiative. Combustion isat superatmospheric pressure and the hot flue gas is expanded throughturbine 52 driving compressor 56. Gas leaving at 50 is fed at 118 to thetop portion 112 of B and reacted with air fed at 120. A flame is formedand the hot gases are brought to equilibrium at for example 900°-1000°C. over secondary reforming catalyst bed 113, whereafter it forms theheat source for the first stage of reforming in B. After cooling in heatexchange zone 124 aided by packing 130 the gas leaves by 132, whence itis passed to further heat recoveries and to steps of shift, CO₂ -removaland fine purification to give ammonia synthesis gas.

Vessels 10 and 110 are preferably provided each with a jacket (notshown) through which cool air or water is circulated, to keep down thetemperature of the pressure-resisting shell and maintain its internalrefractory lining in compression. When jacket cooling is by air, theresulting warm air can be used, preferably after further warming, ascombustion air for burner 16.

The process for producing raw hydrogen or a raw synthesis gas forconversion to organic compounds according to the second process aspectof the invention can follow path Q in the apparatus, using apparatus Bonly. The process is the same as in the combination as far as point 201,except that the methane content of the gas leaving at 150 is preferablyin the range 0.2 to 5% ^(v) /v on a dry basis, and that, if the startinghydrocarbon feedstock is hydrogen-rich, the feed at 146 may includecarbon dioxide. Following path Q from point 201 the gas flows throughline 118 and enters mixer/burner zone 116, in which it reacts withoxygen or possibly enriched air fed in by line 120. The resulting heatevolution is sufficient to reform the methane present in the gas at 118and to heat tubes 128, without the external heat required (apparatus A)when making ammonia synthesis gas. After cooling in heat exchange zone124 aided by packing 130 the gas leaves by 132. Thereafter it is passedto further heat recoveries and to steps of shift, CO₂ -removal and finepurification to give hydrogen, or to cooling and water removal to givesynthesis gas.

In an alternative process for producing raw hydrogen or a raw synthesisgas path R in apparatus A is followed. The starting mixture ofhydrocarbon and steam, possibly with CO₂, is fed at point 202 and reactsas described above, leaving finally at point 203, whence it undergoesheat recoveries and process steps as already described. If ammoniasynthesis gas is required, a conventional air secondary reformer can beused.

Especially if apparatus A only is to be used, two such apparatuses areused side-by-side, with their combustion sections in series and reactionsections in parallel. Thus line 20 feeds approximately double thequantity of air required in A, line 32 leads to an inlet linecorresponding to 20 in the second apparatus A and the outlet linecorresponding to 32 in that second apparatus A leads to the inlet ofturbine 52. However, line 46 is bifurcated and feeds both apparatuses,and the outlet lines 50 are joined before passing to the next unitdownstream.

The Table (first part) shows temperatures, pressures and gascompositions for the production of raw ammonia synthesis gas usingapparatuses A and B in combination, with non-enriched air fed at 120.The second part shows such data for producing raw methanol synthesis gasusing apparatus B only, with substantially pure oxygen fed at 120. It isevident that by using apparatus B only and feeding non-enriched air at120 a raw ammonia synthesis gas containing nitrogen in excess can beobtained and that to use such a gas in ammonia synthesis a subsequentstep of nitrogen removal, before or after synthesis, will be included.Further, by feeding moderately enriched air at 120 the extent ofnitrogen removal can be decreased, for example to zero when thehydrocarbon feedstock is methane and the oxygen percentage is about 35%^(v) /v.

                  TABLE                                                           ______________________________________                                                 Press                                                                              Gas composition % v/v                                                  Temp.   bar                        N.sub.2 +                           Position                                                                             °C.                                                                            abs.   CO   CO.sub.2                                                                           H.sub.2                                                                            CH.sub.4                                                                           Ar    H.sub.2 O                     ______________________________________                                        Ammonia synthesis gas                                                         146    400     40     --   --    2.0 24.5 --    73.5                          140 inlet                                                                            733     39.5   2.1  5.3  27.5 13.9 --    51.2                          150    592     39     2.1  5.3  27.5 13.9 --    51.2                          40 inlet                                                                             825     38.5   6.1  5.7  41.0 7.3  --    39.9                           50    700     38.2   6.1  5.7  41.0 7.3  --    39.9                          113 inlet                                                                            950     38     8.3  5.0  35.8 0.7  17.3  32.9                          132    550     37.5   8.3  5.0  35.8 0.7  17.3  32.9                          52 inlet                                                                             730     7.0    --   --   --   --   --    --                            Methanol synthesis gas                                                         46    390     12     --   --    0.6 19.9 --    79.5                          40 outlet                                                                            790     11.5   6.2  6.2  43.9 2.5  --    41.2                           50    580     11.2   6.2  6.2  43.9 2.5  --    41.2                          52 inlet                                                                             730     7.0    --   --   --   --   --    --                            ______________________________________                                    

In FIGS. 2 and 3a, 3b heat exchange tube wall 210 is for example a tubeexternally heated in a steam reforming furnace and the unit 212 is madeof stainless steel and carries a coating of alumina and metallic nickel.The unit includes solid portions 212A, which may at the top, bottom andcentre form a complete circle or may be split at 213, such that itsshape is maintained by the resilience of the alloy. From the generallycylindrical walls of the unit, vanes 214 project inwardly to provide aninternal contact surface; and small tongues 216 (FIG. 3a) or ribs 217(FIG. 3b) project outwardly to keep the unit walls at a separation fromheat exchange tube wall 210. Vanes 214 and 216 are formed by stampingfrom the wall metal, and thus leave perforations in the walls. Ribs 217are formed by pressing between toothed and grooved rollers during theshaping operation. Ribs 217 could be formed also on the middle or lowersolid portions 212A or in the intervening portions. Alternatively oradditionally one end of each unit can be necked at 218 to fit into thefull-width end of the next unit above or beneath it, or succeeding unitscan be doubly-necked and of full width. The outer vanes and/or ribs ofsucceeding units need not be mutually aligned. The vanes and/or ribs canbe pitched to set up a helical flow pattern in contact with the wall.

If desired, internal vanes 214 can meet or join in axial space 220. Moreusefully, space 220 can have a cross-sectional diameter 30-80% of thatof unit 212 and can be occupied by a similar unit of such a diameter butof the same general shape as unit 212. In a very useful form of theinvention, also not shown, space 220 can be occupied by a further heatexchange tube, and possibly the bottom of tube 210 can be blind, so thatgas flowing down through tube 210 in contact with units 212 flowsupwards through the axial tube. Upward flow followed by downward flow isequally possible.

The following further alternative combinations of units are envisaged:

(a) a unit as shown but without external projection 216 or 217 havingabove and beneath it a unit of greater diameter occupying more of thewidth of tube 210 or possibly fitting snugly as a result of compressionof that unit to close gap 213. The smaller units in such a combinationcan at their ends lie within the larger units resting on vanes 214.

(b) units as shown but without external projections 216 or 217, eachsuccessive pair separated by a ring having at least 3 externalprojections and out-of-plane projections locating the unitsdiametrally--for example a highly pitched non-slip washer.

FIGS. 4a, b show an alternative unit in which internal and externalpitched vanes have been formed. The outer heat exchange tube having wall410 is blind at is lower end, and inner heat exchange tube suspendedwithin it provides the outlet for the reactants. Unit 412, which can bea single unit equal in length to the heat exchange tubes or can be oneof an assemblage as in FIG. 2 has inward vanes 414 (heavy outlines) butat an angle of 45° and outward vanes 415 punched from its walls and bentalso at 45°, the root of the bend being indicated by the dotted lines.In the unit as shown vanes 414 direct the reactants outwardly, butequally the unit could be inverted to provide direction inwardly, or asingle unit could carry both types of vane, or two types of unit couldbe stacked in alternation. Outward vanes 415 direct the reactantsinwardly from the outer wall: they could extend outwardly far enough tocontact the outer wall, but tongues 416 are provided for correctlocation of the unit.

When reactants pass through the unit they undergo endothermic reactionwhile in contact with it but at short intervals leave the surface andmix with reactants that have entered the unit by way of theperforations, having been reheated at wall 210, 410. In this way a highover-all rate of reaction is maintained. As the reactants passdownwardly their temperature gradually increases as a result of heat fedin from the furnace surrounding tube 210, 410 until sufficientconversion has taken place. If an axial tube such as 424 is present, thehot converted reactants flowing upward through it give up heat to thereactants flowing downwardly over units 212, 412 in the annular spacebetween the tubes.

In experimental trials in standard methane steam reforming conditions asdescribed in our European application No. 21736, catalyst units asdescribed in FIGS. 2 and 4, made of stainless steel and carrying acoating of alumina and metallic nickel showed activity of the same orderas that of commercial catalyst, but at a much lower pressure drop.

I claim:
 1. A process for the production of a hydrogen and nitrogen containing gas stream suitable for use in the production of ammonia synthesis gas comprising;(i) reacting a mixture of a hydrocarbon feedstock and steam in a first zone containing a steam reforming catalyst to produce a primary reformed gas stream containing hydrogen, carbon oxides, methane and unreacted steam, (ii) partially combusting said primary reformed gas stream with a member of the group consisting of air, and oxygen enriched air, so as to provide a hot, partially combusted, gas stream, (iii) reacting said hot, partially combusted, gas stream in a second zone containing a secondary steam reforming catalyst so as to decrease the methane content of said gas stream to form a product gas stream, and (iv) simultaneously transferring heat from said primary reformed gas stream leaving said first zone and from said product gas stream leaving said second zone to said mixture while said mixture is in said first zone, so as to supply the heat required for the endothermic reaction in said first zone, and to provide a cooled primary reformed gas stream and a cooled product stream.
 2. A process according to claim 1 carried out at a pressure in the range 10-60 bar abs.
 3. A process according to claim 1 wherein the product gas is subjected to the shift reaction followed by removal of carbon oxides, the proportion of air or oxygen enriched air employed in said partial combustion step being such that, after said removal of carbon oxides, the gas stream has a H₂ :N₂ ratio below 2.5.
 4. A process according to claim 1 wherein the cooled primary reformed gas stream is further cooled, before the partial combustion step, by heat exchange with the mixture of hydrocarbon feedstock and steam to effect preheating of said mixture before it enters said first zone.
 5. A process according to claim 4 wherein(I) said first zone comprises a plurality of annular zones extending into a heat exchange zone from a plenum zone, each annular zone(a) is closed at a first end remote from the plenum zone and opens at a second end into the plenum zone, (b) contains the steam reforming catalyst, and (c) surrounds a tube that is open at the first end of the associated annular zone, and which extends the length of the annular zone and through the plenum zone, said heat exchange zone being downstream of the second zone so that the product gas leaving the second zone passes through the heat exchange zone, whereby each annular zoneis heated from the outside by the product gas steam passing through the heat exchange zone, and is heated from the inside by the primary reformed gas stream leaving that annular zone through the tube associated therewith, and (II) the mixture of hydrocarbon and steam is fed to the plenum zone wherein, prior to said mixture entering said annular zones, it is heated by the primary reformed gas stream flowing through the tubes extending through said plenum zone.
 6. A process for the production of a hydrogen and nitrogen containing gas stream suitable for use in the production of ammonia synthesis gas comprising(i) passing a mixture of a hydrocarbon feedstock and steam through a first zone containing a steam reforming catalyst to produce a primary formed gas stream containing hydrogen, carbon oxides, methane and unreacted steam, (ii) cooling said primary reformed gas stream by heat exchange with said hydrocarbon feedstock and steam mixture while said mixture is in said first zone so as to supply part of the heat required for the reaction in said first zone and to provide a partially cooled primary reformed gas stream, (iii) further cooling said partially cooled primary reformed gas stream by heat exchange with said hydrocarbon feedstock/steam mixture before it enters said first zone so as to effect preheating thereof and to produce a cooler primary reformed gas stream, (iv) partially combusting said cooler primary reformed gas stream with a member of the group consisting of air, and oxygen enriched air, so as to provide a hot, partially combusted, gas stream, (v) passing said hot, partially combusted, gas stream through a second zone containing a secondary steam reforming catalyst so as to decrease the methane content of said gas stream to form a product gas stream, and (vi) cooling said product gas stream by heat exchange with said mixture of hydrocarbon feedstock and steam after the preheating of said mixture and while said mixture is in said first zone so as to supply the remainder of the heat required for the reaction in said first zone and to provide a cooled product gas stream.
 7. In a process for the production of a hydrogen and nitrogen containing gas stream suitable for use in the production of ammonia synthesis gas comprising:(i) reacting a mixture of a hydrocarbon feedstock and steam in a first zone containing a steam reforming catalyst to produce a primary reformed gas stream containing hydrogen, carbon oxides, methane and unreacted steam, (ii) partially combusting said primary reformed gas stream with a member of the group consisting of air, and oxygen enriched air, so as to provide a hot, partially combusted, gas stream, (iii) reacting said hot, partially combusted, gas stream in a second zone containing a secondary steam reforming catalyst so as to decrease the methane content of said gas stream to form a product gas stream, and (iv) providing heat for the endothermic reaction in said first zone by transferring heat from said product gas to said hydrocarbon feedstock and steam mixture while said mixture is in said first zone, the improvement comprising transferring heat from said primary reformed gas stream leaving said first zone to said mixture while said mixture is in said first zone simultaneously with the transfer of heat from the product gas stream to said mixture. 